This invention relates to a method of supporting of workpiece during physical vapour deposition (PVD) and physical vapour deposition apparatus including a support for a workpiece.
High powered transistors often use thick aluminium layers as contact layers in order to handle the very high current densities inherent in these devices. Thick Al films are needed to minimise contact resistance and hence switching losses, which can reduce device speeds and efficiency. Typically the devices have a vertical architecture with a source contact comprising one or more aluminium layers 1-20 μ in thickness deposited onto a semiconductor device embedded on a full thickness wafer.
Aluminium and aluminium films may be deposited onto wafers by magnetron sputtering techniques. Thus a DC power supply is connected between an aluminium target and an annular anode ring. When the system is evacuated and the part to be coated is positioned on support below the target, a low pressure discharge (typically at around several mT) is formed and material is sputtered from the target onto the wafer. Using this technique to deposit thick metal coatings can result in significant increases in the temperature of the wafer being coated as heat from the plasma and instant flux of metal ions or neutral molecules is difficult to remove in a vacuum system. However, there are frequently limits to the temperature that the wafer may attain during production and 450° C. is a common ceiling and is a typical maximum temperature for back end of line integration schemes.
In contrast productivity demands high deposition rates, which in turn leads to high DC powers of the target and these high powers result in significant heat load on the wafers.
Most commonly wafer temperature is controlled by gas conduction between the wafer and the wafer support either including the use of an electrostatic clamp or a mechanical clamping system. Whilst both these approaches work they can be relatively costly to implement in practice and their performance tends to degrade as sputtered material begins to coat the fixture.